This invention relates to a hydraulic wheel brake system for a wheeled vehicle, a manufacturing machine, a testing machine or other similar machine, which is operated by brake fluid pumped by one or more pumps.
One example of such a hydraulic brake system for a wheeled vehicle as mentioned above is disclosed in Japanese CSC Laid-Open Patent Application No. 9-256960. This system has a brake and a hydraulic brake controller. The brake is operated by brake fluid pressure. The hydraulic brake controller includes (1) a pump device which is connected to the brake and has two pumps located in a parallel row in a circuit and (2) a pump device controller which controls the brake fluid pressure of the brake by controlling rotating conditions of the two pumps included in the pump device.
In this hydraulic wheel brake system, brake fluid discharged from at least one of the two pumps is supplied to the brake, and the brake fluid pressure increases. By switching selectively between an operation-mode and a non-operation-mode of each of the two pumps, a cooperative-pressure-increase-mode in which the brake fluid from both pumps is supplied to the brake or an independent-pressure-increase-mode in which the brake fluid from one pump is supplied to the brake is selected, so that the flow of fluid to the brake is controlled.
This hydraulic wheel brake system does not describe any method for reducing brake fluid in the brake through the pump.
It is an object of the present invention to provide a hydraulic wheel brake system having at least one brake and a hydraulic brake controller comprising a pump device, and a pump and valve controller which can not only increase but also reduce brake fluid pressure by controlling the condition of the pump device.
A brake is operatively connected to one or more wheels installed, for example on a vehicle. Each brake includes a wheel cylinder. A pump device comprising at least a pump discharges brake fluid drawing from a reservoir and sends the brake fluid to the brake for brake operation. Each pump is thus hydraulically connected to the brake. The pump device can select a pressure-increasing-mode for pumping the brake fluid from at least one pump to the brake, or a pressure-reducing-mode for discharging the brake fluid from the brake through at least one pump. The pump and valve controller included in the hydraulic brake controller controls the pressure of the brake fluid, flowing to the brake by controlling the rotating condition of the pump or rotating conditions of the pumps.
During normal braking, the brake requires low pressure and high volume brake fluid. During emergency braking, the brake requires high pressure and low volume brake fluid after the brake fluid pressure becomes rather high. If the pump device consists of two pumps, one of the pumps specifically bears a task of low-pressure-high-volume pumping and the other bears a task of high-pressure-low-volume pumping. If the pump device consists of only one pump, the pump has both functions of low-pressure-high-volume pumping and high-pressure-low-volume pumping.
In the pressure-increasing-mode during the braking, the low-pressure-high-volume pump works to supply brake fluid pressure to the brake. In the pressure-reducing-mode, the low-pressure-high-volume pump also works in the reverse condition, sucking brake fluid from the brake to the reservoir. When the high brake fluid pressure is required during the emergency braking, the high-pressure-low-volume pump works.
The reversible condition is achieved by selecting either a normal or reverse rotation direction of a rotary-type pump. It is also achieved by rotating a pump in the normal direction while changing brake fluid passages by providing turn switch valves.
A gear-type of the pump is adopted for the low-pressure-high-volume and reversible pump because it can rotate in both the normal and reverse directions. A vane-type pump is also available. Even if it is a plunger-type pump, it is also available if the hydraulic brake controller uses the turn switch valves as mentioned above. Although a plunger-type pump is adopted for the high-pressure-low-volume pump, other types of pump are also available.
In the pump device having two pumps mentioned above, the low-pressure-high-volume pump is reversible and the high-pressure-low-volume pump is not reversible. But other types of pump devices are also available such as a pump device comprising reversible pumps, a pump device comprising a low-pressure-high-volume non-reversible pump and a high-pressure-low-volume reversible pump, or a pump device comprising two non-reversible pumps.
The brake fluid pressure is controlled by selecting one of the two pumps if the pump device has two pumps. Furthermore the speed of increasing or reducing the brake fluid pressure is controlled by controlling the rotating speed of the pumps or pump.
The pump device is connected to each brake and controls the brake fluid pressure of each brake together in common. Therefore this hydraulic wheel brake system has an advantage of avoiding differences in brake fluid pressure of each brake.
In the case of the pump device with two pumps, the pump device includes a flow preventing valve. The flow preventing valve is located in a passage between the reversible pressure reducing pump, and the brake. It is used to select a connection-mode connecting the reversible pump to the brake or a preventing-mode preventing brake fluid from flowing from the brake to the pump. The pressure increasing pump is connected in parallel with the pressure reducing pump. When the flow preventing valve is determined to be in the preventing-mode, the brake fluid is prevented from flowing out from the brake via the pressure reducing pump. Consequently the brake fluid pressure of the brake does not reduce.
The pump device is connected to a reservoir (this reservoir is also called a master reservoir) for holding brake fluid, and a supplementary reservoir is located between the reversible pump and the reservoir. The brake fluid flows from the brake by way of the reversible pump. A check valve located in a fluid passage between the reservoir and the supplementary reservoir prevents the brake fluid from flowing from the reversible pump to the master reservoir while the opposite flow direction is allowed. The supplementary reservoir holds the discharged brake fluid from the wheel cylinder in the brake. So, until the supplementary reservoir is filled fully, the reversible pump can reduce the brake fluid pressure of the brake. Besides, the supplementary reservoir holds the brake fluid leaked from the reversible pump. Consequently, when the supplementary reservoir is full, the leaked fluid flow stops. That is, a maximum volume of the leaked fluid is limited by the maximum volume of the supplementary reservoir.
The hydraulic brake system has a master cylinder supplying fluid pressure responsive to the force applied to the brake pedal. The pump device is connected to the brake and not connected to the master cylinder. The normal-open solenoid valve is located in the way of the fluid return passage which links the reservoir with the brake controlled by pressure of the pump device without passing through the pump device. A normal-open solenoid valve is located in the fluid return passage for cutting the flow of the fluid return passage when an electric current is supplied and for allowing the brake fluid to flow in the fluid return passage when an electric current is not supplied.
As mentioned above when an electric current is not supplied, the brake fluid flows back to the master reservoir passing through the fluid return passage. Then the brake fluid pressure of the wheel cylinder is reduced. So this normal-open solenoid valve is also called a pressure reducing valve. The normal-open solenoid valve has an advantage that after the braking the brake fluid pressure of the wheel cylinder becomes zero and friction of the brake does not occur.
An open solenoid valve, which the normal-open solenoid valve belongs to, has a valve seat, a valve spool located so that the valve spool can move close to or away from the valve seat, a spring which biases the valve spool away from the valve seat, and an electric driving device providing an electric force in the opposite direction against the direction of the spring force.
The hydraulic brake controller also has a normal-close solenoid valve which is connected in the fluid return passage for sending the brake fluid in the fluid return passage when an electric current is supplied, and for cutting the flow of the fluid return passage when an electric current is not supplied. If the pressure reducing valve is a normal-close valve, the brake fluid pressure of the wheel cylinder controlled by the master cylinder is effective during braking even though there is a failure in the electric circuit. This is so because the normal-close solenoid valve is closed in such a case and the brake fluid of the wheel cylinder can""t flow through the normal-close solenoid valve in the fluid return passage.
The pressure increasing valve and pressure reducing valve comprise a pressure-control-valve device which is located between the brake and the pump device and between the brake and the master reservoir, and used to connect the brake to the pump device or the master reservoir. By controlling the pump device, the brake fluid pressure of a plurality of brakes is controlled in common, and desirable braking performance is obtained. If an anti-lock control, a traction control, or a vehicle stability control is required, it is achieved by controlling the brake fluid pressure of each wheel cylinder on the basis of the pressure supplied by the pump device.
As mentioned above one advantage of this invention is that the brake fluid pressure is controlled precisely and smoothly, because the reversible pump reduces the brake fluid pressure precisely and smoothly. And another advantage is that durability of the solenoid valves is high because the pump device bears the task of the control of the brake fluid pressure rather than the solenoid valves.